They didn't mention which molybdenum compound, so I'll guess it's either molybdite or ferrimolybdite. While rare on Earth, both appear in quartz - something that you can see everywhere in our surface. One doesn't need to claim life appeared in Mars, even if Mo VI presence was proved as necessary condition for the life beginning.

About water being corrosive to RNA, well, there are drier environments here, too.

Added complexity without a good reason/evidence backing it up always smells bad in a hypothesis.